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Sabrekakkonen

Canopy high speed stall

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Sabrekakkonen

I mean that canopy stalls in high speed bacause angle of attack change so fast?



To try to be more precise,

a) It would be questionable whether the canopy would stall DUE to the angle of attack changing too FAST.
(Air reacts very fast to changes so a sudden change in wing shape on our timescale shouldn't make it harder for the air to adhere to a wing surface.)

b) But it will stall if the angle of attack is too HIGH.

c) And if input is applied too fast, it can be easier to get to high an angle of attack, and stall the canopy, before the canopy has time to react and change its pitch and adjust to the new input.
(For example, with front risers, pulling them too suddenly on a canopy where they are light, can cause a problem by getting the nose to zero angle of attack, even if the hand position is no lower than what is OK when front risering more slowly)

In any case, rear risers don't cause as much pitching motion as using toggles, and the distance one has to pull rear risers until the canopy stalls is much lower than for toggles.

So it is easy to stall a canopy on rears, and they are less useful for pulling out of a dive. That's why they joke "always trust your rears", because in reality, you shouldn't, when trying for a sudden dive recovery.

Similarly, people have stalled their non-swooping canopies when trying to land entirely on rear risers. That isn't because they pulled too FAST, but they pulled too FAR (and if you are pulling fast, it is easy to go too far).

When I was new at swooping (and the swoop course was over land) I recall my FX 88 bucking wildly as I was on the edge of, or partially in, a rear riser stall, because I was trying to pull out too quickly on rears. All was OK when I transitioned to toggles.

Rear riser stalls tend to be more benign than toggle stalls, in that the canopy doesn't tend to collapse nearly as much. One isn't dragging the tips down & in with full arm movement. So the transition between stalled and unstalled is not as violent, making it easier to transition back from a stall. Still, if done at high speed, any stall is going to be more violent than at slower speed.



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Aside about definitions of angle of attack:

Rears are a little confusing when it comes to 'angle of attack' as they aren't really changing the angle of attack relative to the original location of the airfoil, but are only changing the airfoil shape. After all, in aero engineering, if one deflects an aileron or flap, one doesn't redefine the trailing edge of the airfoil to be where the deflected flap is. One still defines the angle of attack relative to the undeflected airfoil. But I don't expect skydivers to follow the exact same conventions, so I'm ok with the idea of saying that pulling on rears changes the angle of attack. It does if one is defining the airfoil zero angle as being between the nose and the deflected tail.

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pchapman

After all, in aero engineering, if one deflects an aileron or flap, one doesn't redefine the trailing edge of the airfoil to be where the deflected flap is. One still defines the angle of attack relative to the undeflected airfoil.



The common definition of AoA taught during private pilot training is that it's the angle between relative wind and the chord line, with the latter defined as a straight line from the leading to the trailing edge of the wing. The would include flaps, ailerons, and any other movable surfaces.

During my aerobatic training, I was introduced to another definition of AoA as the angle between relative wind and the zero-lift line of the wing. Some wing shapes continue to generate lift at negative AoA with respect to the chord line because the chord is just a useful approximation of the wing's overall shape. The zero-lift line is the sum total of all aspects of the wing, which again would include any movable surfaces. You can find the zero-lift line by putting the plane into a vertical climb until it has no horizontal movement.

Way more technical information here: http://www.av8n.com/how/htm/aoa.html

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Sorry everyone else, here's my big reply:

mxk


The common definition of AoA taught during private pilot training is that it's the angle between relative wind and the chord line, with the latter defined as a straight line from the leading to the trailing edge of the wing. The would include flaps, ailerons, and any other movable surfaces.



I'll have to disagree there. Yes that is the normal definition, you are quite right. However, for wind tunnel studies or simulations or whatever, everything is calculated relative to the wing with zero deflection on a control surface.

That way one can say for example, "OK, this wing is at 2 degrees angle of attack. Let's graph the lift changes when we deflect the aileron 5 degree." You get the new lift values and plot the value for 2 degrees. So you see how the wing acts relative to the base case.

If you redefined the angle of attack each time, then you'd have to rotate the model in your real or simulated wind tunnel. To give an example with made up numbers: "Let's see, the aileron is 20% of the chord length, and a 5 degree deflection down now puts the trailing edge 3.4% of the chord down and 0.4% of the chord forward from where it was, which corresponds to a 1.6 degree change in the angle of attack, so we are really measuring not for 2 degrees angle of attack but 3.6 degrees relative to the new reference line."

Anyway, it would get quite messy.

So again, the definition is correct as you were taught... But to make things practical, in any aerodynamics textbook or paper, the reference line is not changed when a control surface is deflected.

In skydiving, I guess we can talk about it in either way because there's no mandatory aerodynamics training:

a) Not very technical , but is still useful for practical understanding:
"Dude, I pulled down on the toggles, so the back of the canopy went down, so the angle of attack went way up... and the canopy created a lot more lift"

or

b) The technical way: "The airfoil with toggles pulled, is still defined as being at the same angle of attack. But due to the increased curvature, it is generating a lot more lift at that angle."


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During my aerobatic training, I was introduced to another definition of AoA as the angle between relative wind and the zero-lift line of the wing.



Yes indeed. Normally one uses "leading edge to trailing edge" as the zero reference line, but one can also do it relative to the whatever angle actually gives gives zero lift for that airfoil or wing.

That can be handy to use as a definition when talking with skydivers, as it is more intuitive to understand that "the lift goes negative when the angle of attack goes negative", rather than having to explain why there still may be positive lift at supposedly zero angle of attack, that was defined along some easy to see line between points on the airfoil.

In skydiving I also sometimes see the zero angle of attack line done along the bottom surface of the airfoil -- because it often happens to be fairly flat and the "nose is cut off" for a typical skydiving airfoil. That's handy when doing diagrams.

If one draws the line in conventional airfoil fashion, from the very nose to tail, which will be from the cutoff point at the top of the nose inlets, that is technically correct too, although would tend to result in angle of attack numbers that are hard to compare to an airplane's airfoil. (Axis Flight School used that definition, I saw in a diagram of theirs.)

These differences are often glossed over, both in instructional material and to some degree by myself, when explaining aerodynamics to skydivers. Sometimes that avoids confusion, but at other times a fuller explanation is required to avoid confusion when getting into more detail.

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pchapman,
You are confusing "angle of attack" and "angle of incidence."
AoA relates to the angle the relative wind strikes the wing.
OTOH AoI compares the wing chord with another part of the airframe. Parachute manufacturers tend to refer to AoI as "trim angle" which is defined by the length of suspension lines.
Both are measured relative to the wing chord. Chord is an imaginary straight line between the leading edge and the trailing edge. Since the leading edge is chopped off of most Jalbert (ram-air) canopies, it is simpler to measure the bottom skin to define trim angle.

When flying an airplane, lowering simple flaps lowers the trailing edge, changing camber (curvature) and angle of attack and AoI. AoA is still defined by the angle the relative wind strikes the chord. Even when camber is increased, chord is still a straight line between the leading edge and the trailing edge of the flap. To maintain the same airspeed, the pilot has to change pitch (nose up or nose down) to maintain the same airspeed. Simple flaps slightly increase lift, but their main function is increasing drag to allow a steeper (nose down) landing approach.

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riggerrob


Chord is an imaginary straight line between the leading edge and the trailing edge. Since the leading edge is chopped off of most Jalbert (ram-air) canopies, it is simpler to measure the bottom skin to define trim angle.



Yes, and in the same way, it is simpler to define the angle of attack using that same bottom skin.

Of course, to measure an angle, one has to look at that line relative to something else. For trim angle (or angle of incidence) that is the horizon. For angle of attack, it is the relative wind.

So we should be on the same page.

Quote


When flying an airplane, lowering simple flaps lowers the trailing edge, [....]Even when camber is increased, chord is still a straight line between the leading edge and the trailing edge of the flap.



Yes in one sense it is, sure, one can choose that as one's convention.

But the standard aerospace usage is different. You'll never find any engineering publication about the characteristics of airfoils that redefines the chord line (and thus angle of attack reference point) every time a control surface deflects. It would be a nightmare of changing references. One wants to know the angle the air is coming at the airfoil relative to some fixed reference angle through the airfoil.

I guess if I were describing what happens when a jumper pulls down the brakes, I would say that the increased camber, especially at the aft end, will result in increased lift. The angle of attack does not change until the canopy pitches or otherwise changes its flight path. But for a skydiver trying to imagine what's happening, yes one can think of it as if the angle of attack has suddenly increased, since the line from nose to tail had changed.


These kind of discussions do get at the usages of language and what terms mean and what is considered proper in what communities.

We get "porosity" checks on canopies when technically someone can always chip in to say that "permeability" is the proper term.

A skydiver might think of Lift as an upwards force, when the proper definition of lift is the force perpendicular to the line of flight... which for a descending skydiver is tilted somewhat away from the vertical.

We always have some messiness when it comes to common usage versus technical terminology, and there's no single perfect answer.

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